1. Field of the Invention
This invention generally relates to duplexers and ladder type filters, and more particularly, to a duplexer and a ladder type filter in which an inductor is connected in series with a parallel resonator or an inductor is connected in parallel with a series resonator.
2. Description of the Related Art
In recent years, mobile telephones and mobile information terminals have become widespread rapidly, with the advancements of mobile communications systems. For example, the mobile telephone terminals communicate at high-frequency bands such as 800 MHz to 1.0 GHz and 1.5 GHz to 2.0 GHz. A device for the mobile communication system often employs a high-frequency filter having a resonator or antenna duplexer having the high-frequency filter.
As a resonator for use in the afore-mentioned filter, there are Surface Acoustic Wave (SAW) Resonator and Film Bulk Acoustic Resonator (FBAR). The configuration of the above-described resonators will be described. FIG. 1A is a cross-sectional view of an FBAR. On a cavity 58 in a substrate 50 (an example is a silicon substrate), there are deposited: a lower electrode film 52; a piezoelectric film 54; and an upper electrode film 56. Aluminum nitride, for example is used for the piezoelectric film 54. FIG. 1B is another cross-sectional view of the FBAR. On the substrate 50, there is formed an acoustic multilayer film in which high acoustic impedance layers 62 and low impedance layers 60 are alternately deposited. There are also deposited thereon: the lower electrode film 52; the piezoelectric film 54; and the upper electrode film 56.
FIG. 2 is a top view of a SAW resonator. On a piezoelectric substrate 70, there are provided: comb-like electrodes (also known as Interdigital Transducer: IDT) connected between an input terminal In and an output terminal Out; and reflectors R0 arranged at both sides of the IDT. The IDT and the reflectors R0 are made of a metal, for example, such as aluminum (Al) or the like. Here, in the drawing, the number of the electrode fingers of the reflectors R0 and those of the IDT are shown smaller than the actual one.
A ladder type filter, in which one-port resonators are connected in series and in parallel, is used for a high-frequency filter. FIG. 3 shows a configuration of the ladder type filer. Between the input terminal In and the output terminal Out, series resonators S1, S2, and S3 are connected in series and parallel resonators P1 and P2 are connected in parallel. Referring to FIG. 4A through FIG. 5B, the operation principle of the ladder type filter will be described. The ladder type filter can be separated into series resonators and parallel resonators. Referring to FIG. 4A, in a series resonator, assuming that a resonator S21 is a one-port resonator, one of two signal terminals is set to the input terminal In and the other terminal is set to the output terminal Out. Referring to FIG. 4B, in a parallel resonator, assuming that a resonator P21 is a one-port resonator, one of the two signal terminals is connected to a ground terminal and the other terminal is connected to a short-circuit line.
FIG. 4C shows passband characteristic from the input terminal In to the output terminal Out of the series resonator and parallel resonator. The horizontal axis represents frequency, and the vertical axis represents band-pass amount. The passband characteristic of the series resonator is indicated by a solid line, and that of the parallel resonator is indicated by a dashed line. The passband characteristic of the series resonator has one resonance point (resonance frequency) frs and one antiresonance point (antiresonance frequency) fas. The band-pass amount is the highest at the one resonance point frs, and is the lowest at the one antiresonance point fas. On the other hand, the passband characteristic of the parallel resonator includes one resonance point frp and one antiresonance point fap. The band-pass amount becomes the lowest at the one resonance point frp, and becomes the highest at the one antiresonance point fap.
FIG. 5A shows a structure of a one-stage ladder type filter. Referring to FIG. 5A, a series resonator S22 is connected in series between the input terminal In and the output terminal Out, and a parallel resonator P22 is connected between the output terminal Out and ground. At this point, it is designed that the resonance point frs of the series resonator is substantially identical to the antiresonance point fap of the parallel resonator. FIG. 5B shows passband characteristic from the input terminal In to the output terminal Out of the one-stage ladder type filter. The horizontal axis represents frequency, and the vertical axis represents band-pass amount. With the structure shown in FIG. 5A, the passband characteristic of the series resonator S22 and that of the parallel resonator P22 are combined, and the passband characteristic of FIG. 5B is obtainable. The band-pass amount is the highest around the resonance point frs of the series resonator and the antiresonance point fap of the parallel resonator, and is the lowest at the antiresonance point fas of the series resonator and the resonance point frp of the parallel resonator. The passband is a frequency range that ranges from the resonance point frp of the parallel resonator to the antiresonance point fas of the series resonator, and the attenuation range is the frequency range equal to or lower than the resonance point frp of the parallel resonator and equal to or higher than the antiresonance point fas of the series resonator. In this manner, the ladder type filer functions as a band-pass filter.
There has been proposed a duplexer with the use of a filter having the above-described resonators. The duplexer employs two band-pass filters to arrange a transmit filter between the transmitting terminal and the antenna terminal and arrange a receive filter between the receiving terminal and the antenna terminal. A matching circuit (an example is a phase shifter) is also arranged between the antenna terminal and the transmit filter or between the antenna terminal and the receive filter. The duplexer has functions of outputting a transmitting signal supplied from the transmitting terminal, from the antenna terminal, and outputting a received signal supplied from the antenna terminal, from the receiving terminal.
A description is given of the functions of the matching circuit in a case, for example, where the matching circuit is arranged between the antenna terminal and the receive filter. The matching circuit is used for increasing the impedance of the receive filter as much as possible in the frequency range of the transmitting signal, when viewed from the antenna terminal. This can prevent the power of the transmitting signal supplied from the transmit terminal from entering the receive filter.
In the duplexer for use in the transmit filter, for example, it is necessary to reduce the insertion loss while the transmitting signal is passing from the transmitting terminal to the antenna terminal, and it is also necessary to ensure the attenuation in the passband of the receive filter. It is similar in the duplexer for use in the receive filter. In order to realize a high-performance duplexer, the ladder type filter is employed. The ladder type filter is capable of widening the passband at a relatively low insertion loss, easily obtaining a high attenuation around the passband, and having high power durability. However, higher performance is increasingly demanded for the duplexer. Accordingly, there have been disclosed the following techniques to fulfill the demands for the lower insertion loss and higher attenuation.
FIG. 6A shows a configuration of the filter of conventional technique 1 disclosed in FIG. 12 of Japanese Patent Application Publication No. 09-167939 (hereinafter, referred to as Document 1). In the ladder type filter that includes the series resonators S1 through S3 and the parallel resonators P1 and P2, an inductor L3 is connected in parallel with the series resonator S3 arranged on the side of the output terminal Out. This can increase the out-of-band attenuation on the high-frequency side.
FIG. 6B shows a configuration of the filter of conventional technique 2 disclosed in FIG. 3 of Japanese Patent Application Publication No. 2004-135322 (hereinafter, referred to as Document 2). In the ladder type filter that includes the series resonators S1 and S2 and the parallel resonator P1, the inductors L1 and L2 are respectively connected in parallel with the series resonators S1 and S2, and an inductor LP1 is connected between the parallel resonator P1 and ground. The resonance point and antiresonance point can be adjusted according to the inductances of the inductors L1 and L2.
FIG. 7 shows a configuration of the filter of conventional technique 3 disclosed in FIG. 1 of Japanese Patent Application Publication No. 2003-332885 (hereinafter, referred to as Document 3). The duplexer includes a transmit filter 10b (ladder type filter) having the series resonators S1 through S3 and the parallel resonators P1 and P2 between an antenna terminal Ant and a transmitting terminal Tx. The duplexer also includes a receive filter 20b (ladder type filter) having the series resonators S1′ through S3′ and the parallel resonators P1′ and P2′ between the antenna terminal Ant and a receiving terminal Rx. The duplexer further includes a matching circuit 30 having a capacitor C01 and an inductor L01 arranged among the transmit filter 10b, the receive filter 20b, and the antenna terminal Ant. In addition, an inductor L3 is connected in parallel with the series resonator S3 on the side of the transmitting terminal Tx of the transmit filter 10b, and an inductor L2′ is connected in parallel with the series resonator S2′ arranged in the middle of the receive filter 20b. As stated, in the conventional technique 3, inductors are respectively connected in parallel with the resonators, which are different from those arranged on the side of the transmit filter 10b or the receive filter 20b. In this manner, the excellent loss and out-of-band attenuation are retained.
A description is given of the ladder type filter of conventional technique 4 disclosed in FIG. 2 of Japanese Patent Application Publication No. 2004-173245 (hereinafter, referred to as Document 4). According to FIG. 2 of Document 4, a first inductor L1 is connected in series with a parallel resonator 5, and a second inductor L2 is connected in parallel with a series resonator 7. The resonance point of the parallel resonator 5 is shifted to a lower side since the first inductor L1 is connected in series with the parallel resonator 5, and the antiresonance point is produced at a lower side of the resonance point of the series resonator 7 since the second inductor L2 is connected in parallel with the series resonator 7. The afore-described resonance point and the afore-described antiresonance point are designed to be substantially matched. This increases, in particular, the attenuation in the stopband at a lower side, which is the opposite side of the passband.
A description is given of the ladder type filter of conventional technique 5 disclosed in FIG. 6 of Japanese Patent Application Publication No. 2002-223147 (hereinafter, referred to as Document 5). According to FIG. 6 of Document 5, an inductor is connected in series with the series resonator, and another inductor is connected in parallel with the series resonator. Two parallel resonators are commonly connected on the side of ground, and then the inductor (for attenuation pole L) is connected in series with ground. In this manner, the frequency of the attenuation pole around the passband is adjusted.
A description is given of the ladder type filter of conventional technique 6 disclosed in FIG. 2 of Japanese Patent Application Publication No. 10-313229 (hereinafter, referred to as Document 6). According to FIG. 2 of Document 6, there is disclosed the duplexer where an inductor Lp for matching is connected between the common terminal Ant and ground.
It is to be noted that in conventional technique 1, however, there is no consideration of the characteristic of a duplexer that includes a filter, and there are no specific measures to improve the characteristic of the duplexer. In conventional technique 2, the inductors are connected in parallel with all the series resonators, and the inductors are connected between all the parallel resonators and ground. When the duplexer is composed with such configuration, the attenuation can be increased in the opposite bandwidth (for example, the receive bandwidth of the transmit filter). However, the attenuation will be degraded drastically in a wide bandwidth. In the conventional technique 3, the inductor connected in parallel with the series resonator is capable of increasing the attenuation of the opposite bandwidths of the filters 10b and 20b. However, the matching circuit 30 is necessary on the side of the antenna terminal Ant of a transmit filter 10b or a receive filter 20b. In such technique, two elements of the inductor L01 and the capacitor C01 are used, thereby making it difficult to reduce the mounting area, in other words, difficult to downsize the duplexer.
In Wideband Code Division Multiple Access (W-CDMA)/Universal Mobile Telecommunications System (UMTS), the market is rapidly expanding and there is a demand for a duplexer of a lower loss, higher isolation, and higher attenuation in a wide band in both transmit and receive bandwidths than those of the conventional duplexers. In such system, there are characteristics that the transmit bandwidth ranges from 1920 MHz to 1980 MHz, the receive bandwidth ranges from 2110 MHz to 2170 MHz, and there is a wide gap between the transmit bandwidth and the receive bandwidth, whereas there is a narrow gap, only 20 MHz, between the transmit bandwidth and the receive bandwidth, for example, in PCS system (the transmit bandwidth ranges from 1850 MHz to 1910 MHz and the receive bandwidth ranges from 1930 MHz to 1990 MHz) or in Cellular system (the transmit bandwidth ranges from 824 MHz to 849 MHz and the receive bandwidth ranges from 869 MHz to 894 MHz).
In the conventional ladder type filters, however, the attenuation around the passband is easily obtainable with the use of the attenuation pole of resonance in the parallel resonator or that of the antiresonance in the series resonator, as described above. Meanwhile, in W-CDMA/UMTS system, it is difficult to obtain the attenuation in the frequency range (the receive bandwidth of the transmit filter and the transmit bandwidth of the receive filter) widely apart from the passband.
The problems of the conventional techniques will be discussed in the following. In the conventional techniques 1 and 3, the inductor is connected in parallel with the series resonator. However, no inductor is connected to the parallel resonator and the attenuation pole of the resonance in the parallel resonator exists around the passband, thereby making it impossible to ensure the attenuation sufficiently in the frequency range widely apart from the passband.
In the conventional techniques 2 and 4, the circuit is configured such that the inductor is connected in parallel with the series resonator, and the inductor is connected in series with the parallel resonator. However, as shown in FIG. 3 of Document 2 or FIGS. 22 and 23 of Document 4, an inductor having a relatively large value is needed in order to produce the attenuation pole of the resonance to the frequency range widely apart from the passband in the circuit where the inductor is connected in series with one parallel resonator. For this reason, it is difficult to form the inductor with the use of a line pattern in a package in order to reduce the size of the package. Also, as shown in FIG. 23 of Document 4, in a case where there are two circuits, each of which includes the inductor connected in series with the parallel resonator, there are problems: downsizing is more difficult; and the characteristic is degraded due to the electromagnetic coupling between the two inductors.
In the conventional technique 5, as shown in FIG. 1 of Document 5, the ground sides of the two parallel resonators become common, and then the inductor for the attenuation pole is connected in series therewith. This produces an advantage of making the value of the inductor relatively small. Nevertheless, in the conventional technique 5, as described in paragraphs 0081 through 0091 of document 4, the attenuation pole generated by the connection of the inductor for the attenuation pole is widely deviated from the attenuation pole of the antiresonance generated by the inductor and the capacitor connected in parallel with the series resonator. Therefore, no improvement can be seen greatly in the attenuation in both the lower frequency side and the higher frequency side of the passband as shown in FIG. 10 or 11 of the conventional technique 5, when the characteristic of a case 1 where only the inductor for the attenuation pole is connected is compared with cases 2 and 3 where the inductor and capacitor are connected in parallel with the series resonator.
In the conventional techniques 1, 2, and 5, there is no consideration to the characteristic of the duplexer that includes two filters. Besides, there are no specific measures that improve the characteristic of the duplexer. In addition, in the circuits shown in the FIG. 3 of Document 2, FIGS. 22 and 23 of Document 4, and FIGS. 1 and 6 of Document 5, the inductors are connected in parallel with all the series resonators, and the inductors are connected between all the parallel resonators and ground. With such configuration, it is possible to increase the attenuation in the stopband. However, the attenuation in a wide band is degraded drastically.